Technical plasmas are increasingly being made use of in industry and in research laboratories, and are used among other things for producing high-tech products such as microchips in EUV lithography, for cleaning various surfaces and materials in medical physics, and for coating mirrors in the production of optical elements. Technical plasmas include for example hollow cathode discharges, RF discharges, magnetron discharges, corona discharges and linear discharge arrangements. The reflectivity of metal, generally reflective surfaces plays a very important role in the monitoring and checking of certain production processes.
Hereinafter, the reflectivity of surfaces is divided into two subject areas: the reflection of particles (atoms) and the reflection of light (photons). The reflection of the plasma particles on the surface is an irresistible process which takes place as a result of the interaction between the plasma ions and the surface. Depending on the desired technical production processes, this interaction may have positive and negative effects. For example, in the production of the mirrors, the substrates (Fe, W, Mo) are specially coated (Al, Au or Ag). By contrast, the coating with plasma impurities or the erosion of the substrate are negative, generally unintended effects on the surface. The characteristics of the particle reflection, such as energy distribution and angular distribution of the reflected plasma particles, can provide information regarding the state of the surface, such as the type of material or the roughness [1, 2]. Further diagnostic access comes from the spectral reflectivity of optical metal mirrors and reflective surfaces [3]. This can be made use of to check the efficiency of the technical process. For example, various optical systems are made use of so as to quantify the production processes and control them better using cameras and special instruments (spectrometers). Quite often, the plasma parameters are also determined in the process. These also include very complex optical labyrinths consisting of a plurality of mirrors. The most important limiting case of spectral reflection is specular reflection, in other words mirror reflection, or a purely diffuse reflection and the polarization characteristics thereof [4]. Using appropriate theoretical models, other physical parameters of the surface, such as the roughness, can be determined on the basis of the physical variables, such as the energy and the angular distribution of the particles.
So as to obtain quantified predictions as to the progression of the process, it is necessary to determine the reflectivity of the surface on an ongoing basis during the process [3]. The biggest problem in this context is the fact that the material usually has to be removed from the plasma for this purpose, and the plasma operation thus has to be interrupted.
Simultaneous measurements of the particle reflection and light reflection can only be united in a single laboratory experiment at great complexity, since the characteristics of photons and atoms are so different. Different methods are therefore used for determining the particle reflection and light reflection characteristics. Thus far there is no method which makes it possible to determine or measure both parameters, in other words to approximately measure the reflection of particles and of photons in situ in the plasma without additional means.
To determine the energy distribution and angular distribution of reflected atoms on a surface, ion irradiation of the surface is combined either with laser-induced fluorescence (LIF) or with an energy-mass detector. The measuring setup is shown schematically in FIG. 1.
Initially, the ions striking the surface are neutralized. The neutral atoms leave the surface with an energy less than the ion incidence energy (E0). The atoms are backscattered in a particular energy distribution f(E<E0). Observation of the reflection at different angles provides the angular distribution of the reflected atoms. The theoretical description of the particle reflection process can be calculated using Monte Carlo simulations, such as the TRIM or SRIM code [5]. In general, a close correspondence to the measured data is found in this way. At this point, it should now be noted that the roughness of a surface and any coating thereof strongly influence the results of this distribution. As a result, the angular distribution and energy distribution of the scattered particles during the plasma operation are irreplaceable measurement variables [3].
In the case of reflecting surfaces, it is a question not just of the reflected particles, but also of the spectral or specular reflection of light. So as to be able to answer this question, a large number of spectrophotometric instruments are available here. All in all, the measurement of the reflectivity of various materials is one of the most important tasks in spectrophotometry [3]. The sketch in FIG. 2(a) shows the basic progression of measurements of this type, although the details may vary between different instruments [3].
Before placement in the plasma, the mirror is carefully measured for reflectivity in the mirror laboratory. While an absolutely calibrated light source is illuminating the mirror, the reflection is measured at the angles which are subsequently used in the production process. FIG. 2(b) shows how the mirror is subsequently placed in the plasma chamber and the plasma switched on. The production process is monitored by the detector B. As a result of the contact with the plasma, the reflectivity may change over time. Because of the constantly occurring collisions of the plasma particles (or because of the particles of impurities) with the surface of the mirror, in most cases the reflectivity of said surface becomes worse. However, it is not possible to remeasure the reflectivity in situ, since the technical process usually has to be interrupted for this purpose. Instead, the mirror has to be removed from the plasma and tested in the laboratory again, repeating the loop shown in FIG. 1. For targeted coating processes, or else if an operating error occurs, the material has to be removed from the plasma again so as to determine the reflectivity once again.
There are applications in which it is not possible to open the plasma chamber and repeat the measurement of the reflectivity in accordance with FIG. 1. These include processes in toxic or radioactive environments. New measurements of the reflectivity of the surface of particles and photons in environments of this kind may not be possible in practical terms or else simply involve high costs and expenditure of time. Some major projects planned for the future, such as the fusion experiment ITER or the experimental reactor DEMO, will have to deal with radioactively activated materials. For the surfaces subsequently exposed to the plasma, the consequences are even more serious, since information regarding the states of the surfaces (walls, substrates and mirrors) cannot be exactly predicted.